Output a valid XML dateTime for the validUntil metadata property

[agrobbin]

The XML generated prior to this change was considered invalid, according to samltool.com's XML validator. An example metadata XML document:

<md:EntityDescriptor xmlns:md="urn:oasis:names:tc:SAML:2.0:metadata" ID="_a881df95-a3e4-43df-a340-47742de4c356" entityID="..." validUntil="2022-06-15T03:03:01+0000">
<md:SPSSODescriptor AuthnRequestsSigned="false" WantAssertionsSigned="true" protocolSupportEnumeration="urn:oasis:names:tc:SAML:2.0:protocol">
<md:NameIDFormat>urn:oasis:names:tc:SAML:1.1:nameid-format:emailAddress</md:NameIDFormat>
<md:AssertionConsumerService Binding="urn:oasis:names:tc:SAML:2.0:bindings:HTTP-POST" Location="..." index="0" isDefault="true"/>
</md:SPSSODescriptor>
</md:EntityDescriptor>

The +0000 is the culprit, with the following validation error received from samltool.com:

Line: 1 | Column: 0  --> Element '{urn:oasis:names:tc:SAML:2.0:metadata}EntityDescriptor', attribute 'validUntil': '2022-06-15T03:03:01+0000' is not a valid value of the atomic type 'xs:dateTime'.

Additionally, should someone pass a non-UTC time for validUntil, that also produced invalid XML.

Now, we coerce the provided validUntil into a UTC time, and hard-code a Z at the end of the format, to consistently produce valid XML.

[agrobbin]

The error in CI seems to be unrelated to this change (it's a JRuby OpenSSL error).